With the development in recent years of such copying devices as full-color copiers, the quality of the reproduced image has reached the level at which the reproduction cannot be distinguished from the original image (i.e., the document) with the naked eye. This sort of faithful reproduction has become easy to obtain. The development of high-quality copiers has increased the risk that copiers will be used for an improper purpose. Documents which should not be reproduced, such as bank notes or stock certificates, might be copied. Classified documents might be copied and leaked. Various devices have been developed to address this risk.
One such device is the image processor described in the Japanese Patent Publication of Patent 2-210591. This processor works in the following way. The location of the characteristic portion of the bank note or other nonreproducible item (in the patent report, the red seal) is specified. The image data associated with the specified region are compared with previously recorded characteristics (i.e., with a reference pattern). From the goodness of fit of the two patterns (i.e., their degree of resemblance to each other) a determination is made as to whether the document being processed is one which may not be reproduced. If it is determined that this is a nonreproducible document, specified processing is executed to prevent it from being copied. This may be accomplished by interrupting the processing being executed by the copier, printing out a black screen, or printing out the original image with the word "copy" superimposed on it.
Existing image processors capable of detecting nonreproducible documents and preventing their reproduction often attempt to make their detection function fool proof in order to reliably prevent counterfeiting. To this end the threshold value used to make a determination is lowered to the point where some documents which should be reproduced are prohibited. When this happens, the essential function of the copier, which is to reproduce the document with precisely the same dimensions or at a specified magnification, will not be carried out. This has a deleterious impact on the ordinary, law-abiding user.
To prevent such situations from arising, the threshold value used to determine recognition can be set higher. In this case only items identical to the recorded characteristic (i.e., to the reference pattern) or very closely resembling it will be detected. A document that resembles the reference pattern (i.e., a document whose reproduction is not prohibited) will be permitted to be copied. However, if the reference mark is defaced, either on purpose or accidentally, so the degree of resemblance of the mark to the reference pattern is reduced, the mark will not be detected using the higher threshold, and copying will be permitted. For example, someone might cover the entire pattern on a document with black or some other color. If color is one of the criterion used for determination, the document, which now has a different color, will not be recognized as containing the item to be detected, and copying will be permitted.
This situation is not limited to copy machines, but also occurs with scanners and various other processing devices which read an image.